In a gas turbine engine, ambient air is drawn into a compressor section. Alternate rows of stationary and rotating aerofoil blades are arranged around a common axis, together these accelerate and compress the incoming air. A rotating shaft drives the rotating blades. Compressed air is delivered to a combustor section where it is mixed with fuel and ignited. Ignition causes rapid expansion of the fuel/air mix which is directed in part to propel a body carrying the engine and in another part to drive rotation of a series of turbines arranged downstream of the combustor. The turbines share rotor shafts in common with the rotating blades of the compressor and work, through the shaft, to drive rotation of the compressor blades.
It is well known that the operating efficiency of a gas turbine engine is improved by increasing the operating temperature. The ability to optimise efficiency through increased temperatures is restricted by changes in behaviour of materials used in the engine components at elevated temperatures which, amongst other things, can impact upon the mechanical strength of the blades and a rotor disc which carries the blades. This problem is addressed by providing a flow of coolant through and/or over the turbine rotor disc and blades.
It is known to take off a portion of the air output from the compressor (which is not subjected to ignition in the combustor and so is relatively cooler) and feed this to surfaces in the turbine section which are likely to suffer damage from excessive heat. Typically the cooling air is delivered adjacent the rim of the turbine disc and directed to a port which enters the turbine blade body and is distributed through the blade, typically by means of a labyrinth of channels extending through the blade body. Cooling of blade surfaces is aided by impingement cooling wherein small cooling holes extend from the channels through internal walls of the blade body and cause jets of air to impinge on the appropriate surfaces. For example (but without limitation) impingement cooling is often used to cool the leading edge passages of an aerofoil.
Turbine blades are known to be manufactured by casting methods. A mould defines an external geometry of the turbine and a core is inserted into the mould to define the internal geometry, molten material (typically a ferrous or non-ferrous alloy) is then cast between the mould and the core and the core subsequently is removed, for example by leaching. The core can include arrays of pedestals which define the arrangement of cooling holes in surfaces of the turbine blade.
Arrays of holes are designed to provide optimum cooling of a surface. Existing designs feature single rows of holes (an example is disclosed in US 2009/0317258); staggered rows and grid formations. Radial stresses in a blade body are mainly driven by centripetal loads caused by blade rotation. From the perspective of blade design, to minimise stresses in the impingement holes it is advantageous to employ a single row of holes aligned with the radial stress field in the impingement web.